usbredir is the name of a network protocol for sending usb device traffic over a network connection. It is also the name of the software package offering a parsing library, a usbredirhost library and several utilities implementing this protocol.
The protocol is documented here, this document also explains what is meant with a usbhost and a usbguest.
usbredir was created for use with Spice, which is why it is hosted on spice-space.org, but the protocol and the usbredirhost are completely independent of spice, they could for example also be used to create a vnc extension for redirecting usb devices over a vnc connection to a qemu virtual machine.
The usbguest side is currently only implemented for qemu, and shipped as part of qemu (enabling this in qemu requires the libusbredirparser library to be available when building qemu).
Latest version: usbredir-0.4.3.tar.bz2
You can find the official usbredir git repository here.